When New Horizons launched at 2 p.m. Eastern time on January 19, 2006, its first Atlas V stage and solid rocket boosters fell back to Earth within minutes of launch, never entering orbit.

New Horizons then proceeded on to Earth orbit aboard its Atlas V’s powerful Centaur second stage, which then re-ignited to propel itself, New Horizons and its STAR-48 third-stage solid rocket out of Earth orbit.

Just seconds after the Centaur stage completed that Earth-escape maneuver, it was discarded, and New Horizons was propelled onto its Pluto trajectory by a brief (84-second) but powerful (up to 13 G!) burn of its third stage. That derelict third stage is now traveling out of the solar system in the general direction of Pluto, much like New Horizons, though it will miss Pluto by hundreds of millions of miles because it has no ability to make the course corrections to precisely target for Pluto as New Horizons itself has.

But what became of the also now derelict Centaur second stage New Horizons left behind? It’s orbiting between the Earth and the asteroid belt, with a period of 2.83 years, never reaching farther than three times as far from the Sun as the Earth does.

Orbital calculations reveal that the approximate current positions of New Horizons, its STAR-48 third stage, and its Centaur second stage are as shown in the figure below. The Centaur stage is now on its second orbit of the Sun, having just passed its aphelion, or greatest distance from the Sun, and is now approaching the orbit of Mars as it falls back sunward.

– Alan Stern and Yanping Guo

In October 2005, inside the mobile service tower on Launch Complex 41 at Cape Canaveral Air Force Station in Florida, the Lockheed Martin Atlas V Centaur stage for New Horizons is moved into place over the waiting first stage below it. Having done its job to boost New Horizons out of Earth orbit, the now-derelict stage is orbiting the Sun between Earth and the asteroid belt. (NASA photo)